The invention relates to an optical pulse generator, and more particularly relates to an optical pulse generator capable of generating short optical pulses.
The optical pulse generator for generating short optical pulses is necessary for structuring an optical communication system with ultra-high speed and large capacity.
Japanese laid-open patent publication JP A1-H6(1994)-281896 discloses an optical pulse generator which includes two optical modulators of the electroabsorption type which are made from semiconductors and are driven by the voltage of 0(V) or the forward bias voltage and the sinusoidal voltage (referred to as a electroabsorption type optical modulator hereinafter). This optical pulse generator generates optical pulses as follows. The incident light having a constant output level on the optical pulse generator is applied to the first electroabsorption type optical modulator to be modulated therein. Then, the output light from the first optical modulator is applied to the second electroabsorption type optical modulator which is driven by the bias voltage and another sinusoidal voltage which is derived from inverting the phase of the former sinusoidal voltage, so that a time difference is caused between the light modulated by the first and second electroabsorption type optical modulators, respectively, thereby generating short optical pulses having the repetitive frequency two times as high as the oscillation frequency of the sinusoidal voltage generator.
In the optical pulse generator of this type, when the electroabsorption type optical modulator is driven by the sinusoidal voltage, its optical output characteristic is non-linear, so that it can be operated as an optical gate of which the leading and trailing time is short.
In other words, if a value of the bias voltage is properly selected to be 0 (V) or other forward voltage, the time when the optical gate is kept completely opened, can be made more than a half of the repetitive oscillation period of the sinusoidal generator.
Also, the first and second electroabsorption type optical modulators operate as an optical gate, wherein the operational phase of the second modulator is made opposite to that of the first one. When a laser beam having a constant output level enters such first and second optical gates in sequence, the rising and falling portions of the laser pulse produced by the first gate are superposed only on the rising and falling portions of the laser pulse produced by the second gate, thereby enabling short optical pulse to be generated which has the frequency two times as high as the repetitive frequency.
Furthermore, the disclosure teaches that in the electroabsorption type optical pulse generator, the repetitive frequency may be arbitrarily changed by changing the frequency of the sinusoidal voltage generator.
However, the previous optical pulse generator contains the following problems. That is, the sinusoidal voltage has to be separately supplied to respective first and second electroabsorption type optical modulators. Consequently, in order to generate short optical pulses, the optical pulse generator is required to include at least two each of the DC voltage source and the sinusoidal modulation voltage source, a power divider for diving the sinusoidal modulation voltage into two, and an electric phase delay circuit. Therefore, this naturally results in enlargement of the generator scale.
In addition to the above problem, if it is intended to integrate the first and second electroabsorption type optical modulators on a substrate, two each of the high frequency feeder and the terminal resistance for impedance matching have to be disposed on the header for use in mounting elements thereon. This apparently causes various difficulties in design and manufacturer of such header.
In general, the length of the electroabsorption type optical modulator element might be about 300 .mu.m or less. Even when two optical modulators are integrated on the same substrate, the total length of them might be about 700 .mu.m or less. On one hand, in order to avoid having the two high frequency feeders make contact with each other, they would need a space or width of about 1 mm therebetween. Consequently, it is hardly possible to feed the high frequency in the identical direction. Even if it is tried to feed the high frequency in the opposite direction, it would become difficult to ensure the space for disposing the terminal resistance for impedance matching use.
Furthermore, alignment of the optical coupling has to be adjusted at two places, that is, one place is where the incident light enters and the other place is where the light comes out. When modularization is intended, there will be an number of steps for adjusting the alignment of the optical coupling between elements such as lenses. Moreover, a very tiny antireflection film has to be formed on both of the optical end faces of the modulator. This makes the manufacturing process more complex and difficult.